You can use a "positive lookahead" like this:

,(?=.*,)

The lookahead is the part in parens. It basically says "only replace this comma if there's another comma later in the string.

The code would look like this:

echo preg_replace(',(?=.*,)', '', $str);

You can use a "positive lookahead" like this:

,(?=.*,)

The lookahead is the part in parens. It basically says "only replace this comma if there's another comma later in the string.

The code would look like this:

echo preg_replace('/,(?=.*,)/', '', $str);

I tested this with RegexBuddy to confirm it works: